The type of muffler which is the subject matter of this invention can best be understood by specific reference to an environment in which the muffler can reside.
It is known that fluidized bed combustion chambers can burn sulfur type fuels with low pollution emission. Consequently, coal or coke having a high sulfur content can be burned in fluidized bed combustion chambers with the sulfur liberated in combustion being converted to calcium sulfate. Unfortunately, to maintain the fluidized beds in an operative state, high volume, high pressure air flow is required.
The need for this high volume, high pressure air flow can be simply understood. Fluidized beds comprise a solid granular mass held with in a confined bed. Air is introduced from the bottom of the bed and exits from the top of the bed. During its upward flow, the air passes around the particulate matter within the bed, keeping the particles of the bed in a fluidized suspension. Indeed, the mass of the fluid bed looks and acts not unlike a liquid.
Such fluidized beds are particularly advantageous for chemical and combustion processes. In the example here, heat is produced by the combustion while a chemical reaction purges the sulfur from the combustion and discharges the sulfur as calcium sulfate.
Unfortunately, such fluidized beds require high volume, high pressure air flow. In the example here, rotating turbo fan machinery produces air outflow in the range of 40,000 to 80,000 cubic feet per minute at a discharge pressure of 3 to 4 psi. The term high pressure is used in this disclosure to distinguish the more usual case where such rotating turbo fan machinery operates at pressures of about 1 psi.
It will be understood that this muffler is also effective at low pressure, that is, 1 psi and below.
It is known to muffle the inlets to such rotating turbo fan machinery. A more difficult problem is the muffling of the outlet of such turbo fan machinery. The muffler herein can be used at either inlet or outlet.
Although rotating fan turbo machinery constitutes the major use of the muffler herein, the muffler could as well be used with certain piston type installations. Acoustically, the sound output must include relatively narrow band acoustical emissions including a fundamental frequency which is usually directly a function of the turbo fan or piston frequency.
Regarding the outlets of such compressors, these outputs commonly are contained in strong steel ducts leading from the discharge of the turbo fan machinery to the inlet of the fluidized bed. By way of example, 3/8 inch steel duct can be used surrounded by a 4" layer of fiberglass insulation which is in turn covered by a lead sheath. The discharge duct is rather large--being in the order of six feet long and one foot wide.
Even with a duct constructed and sound insulated as described, the sound on the outlet side of the duct can be excessive. In the case of a particular power plant, the sound generated by the rotating turbo fan machinery was so excessive that the entire plant could not be operated--until the muffler of this invention could be designed and installed. The unmuffled noise of the power plant included a low frequency rumble that could almost not be heard, a loud rushing and hissing sound at high level, and a series of other tones and whines that in combination sounded not unlike a series of sirens.
It should be understood that sound propagating through the acoustically insulated discharge duct was not the only problem. Actual damage to the duct work occurred in an operating period that was about 90 days. Fatigue failure of metal with cracking of the duct at the fan discharge occurred. In short, without muffling, such plants can constitute an aggravated noise nuisance with impractical short operating lifetimes of their required duct work.
It will be understood that the muffling requirement is unusual. Conventional mufflers having large pressure losses cannot be used. If substantial pressure loss occurs, insufficient pressure will remain for the fluidized bed operation or alternatively, excessive energy will be consumed in generating the required high volume, high pressure fluid flow.